In the wood construction sector, the reduction of energy costs involves the use of alternative methods to natural drying such as the use of green wood obtained immediately after cutting the tree. However, the high hygrometry coupled with the delayed behavior and the orthotropic / heterogeneous nature of the wood can lead to cracks that eventually weaken the structures, especially when they are more and more massive. In this case, the present study proposes a new method for the measurement of the stress intensity factor and the critical energy release rate G in green wood using X-ray microtomography (µCT). The work was performed on "normal" and "reaction" wood for two species; a hardwood (Populus nigra) and a softwood (Abies alba). After cutting from fresh trees, samples were machined in the form of Mixed-Mode Crack Growth specimens. The specimen was placed in a press to control the advance of the crack tip in opening mode. Then a microtomographic scan allowed to observe and measure the crack front in 3D. The critical energy release rate was obtained verus crack fronts, on the outer faces and perpendicular to the direction of propagation for each type of wood. Results enlightened that G is greater in normal wood than in reaction wood in relation to the density of each species. This work highlights the potential of nondestructive observation of the process zone by μCT in the green wood.